passing little bit of discharge in pregnancy is common, which is called physiological leucorrhea.
It will be mucoid or whitish in color, little in amount without any smell. Whereas there will be increased vascular supply to the cervix and vagina during pregnancy chances are there because of friction or due to some stressful work little bit of spotting can be seen which mixed with mucoid discharge may appeared as brown vaginal discharge
One more chance is presence of blighted ovum
means in the gestational sac
the embryo development will not be there only fluid will be there, when it is going to abort the brownish discharge will come.
In case of missed abortion
also this type of discharge can be seen.
This is only for your information, you need not worry. You visit your gynecologist get examined also undergo ultrasound
examination which will rule out your doubts.